Tetrahedral complexes energies

Thc Crystal l-ield Siabili2ation Energy (CFSl ) is the additional stability which accrues to an ion in a complex, as compared to the free ion, because its d-orbitals are split In an octahedral complex a l2 electron increases the stability by 2/5Ao and an Cf, electron decreases it by 3/5Ao- In a tetrahedral complex the orbital splitting is reversed and an e electron therefore increases the stability by 3/5At whereas a t2 electron decreases it by 2/5Ai. 

In octahedral complexes, the e -orbitals (dz< and dx2 -yi) lie higher in energy than the t2 -orbitals (dxy, dyz, and dzx). The opposite is true in a tetrahedral complex, for which the ligand field splitting is smaller. 

FIGURE 16.28 Tbe energy levels of the d-orbitals in a tetrahedral complex with the ligand field splitting A,. Each box (that is, orbital) can hold two electrons. The subscript g is not used to label the orbitals in a tetrahedral complex. 

The crystal field energy level diagram for tetrahedral complexes. The d orbitals are split into two sets, with three orbitals destabilized relative to the two others. 

From a consideration of the combination of ligand and metal orbitals, it should be apparent that the overlap is much more effective in an octahedral complex (in which orbitals are directed at ligands) than in a tetrahedral complex (where orbitals are directed between ligands). The result is that the energy difference between the e and t2 orbitals in a tetrahedral complex is much smaller than that between the t2g and eg orbitals in an octahedral complex. As we saw when considering the two types of complexes by means of ligand field theory, At is only about half as large as A0 in most cases. 

Determine the ligand field stabilization energy for d°-d10 ions in tetrahedral complexes. Although there are no low-spin tetrahedral complexes, assume that there are. 

A splitting of magnitude A6 is produced and it depends on the nature of both the metal ion and the ligand. In the case of the octahedral field each electron placed in one of the t2g orbitals is stabilized by a total of -2/5A, while electrons placed in the higher energy eg orbitals are destabilized by a total of 3/5A. The splitting for a tetrahedral complex, Atet is less than that for an octahedral one and algebraic analysis shows that Atet is about 4/9A. ... 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions. 

On the second problem the people in my area are responsible for the poor state of affairs. I take most responsibility because outside of octahedral and tetrahedral complexes no complete assignments have been made. But, we do have new results, and I think it is of some interest to present them. Cooper Langford and I at Columbia now have conclusive results on the energy levels in the distorted octahedral cobalt compounds. These are the Co(NH3)5X+2 complexes. 

Ligand-field theory predicted (10, 22) that tetrahedral nickel(II) complexes should be unstable with respect to octahedral ones, at least so long as the two extra ligands were available. This arises because, if one accepts the d-orbital center of gravity as an energy-zero (a point which should be raised more often), the crystal-field stabilization of an octahedral complex works out to be 0.84 A greater than that of a tetrahedral complex with... 

Splitting of the energy levels. Both an octahedral complex (two electrons in orbitals) and a tetrahedral complex (four electrons in orbitals) are less favorable in this case. 

In order to find out the relative energies of the e and t2 orbitals, let us place our tetrahedral complex in a set of coordinate axes as shown in Figure 9.10. Once more, we can make a comparison between either of the e orbitals and any one of the t2 orbitals, and perhaps the best selection in order to visualize the relative electrostatic energies is again the pair dxy and dxi.y The difference here is much less striking than in the octahedral case, but it can be seen that qualitatively an electron in the dxy orbital will have a higher... 

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